Macrophages are important immune cells in the body that regulate different biological processes involved in inflammatory and infectious diseases. Macrophages originate from the mononuclear phagocyte system (MPS) in the bone marrow. The MPS system is comprised of monocytes and tissue macrophages in their various forms. The primary function of monocytes is phagocytosis. Upon exiting the bone marrow, monocytes circulate in the blood and become activated in various tissues. Once the monocyte reaches the extravascular tissue, it matures into a larger phagocytic cell called a macrophage. They reside in the liver (Kupffer cells), lungs (alveolar macrophages), spleen, lymph nodes, thymus, gut, marrow, brain, connective tissue and serous cavities.
In order to combat inflammation and infections, macrophages are converted from the rested to an activated state. The activated cells have an enhanced capability to attack and kill infecting pathogens and tumor cells.
The active state is marked by an increase in size and the development of more psuedopods (cellular projections). The cytokines released from T-lymphocytes such as interferon-gamma (IFN-γ), tumor necrosis factor-alpha (TNF-α) and granulocyte-monocyte colony stimulating factor (GM-CSF) can act as signals to activate macrophages. Additionally, the activating signal can come from microbial products such as lipopolysaccharides, immune complexes, chemical mediators, and extracellular matrix proteins including fibronectin.